A ground fault in an electric machine that is connected to an electrical network may pose a considerable danger for continued operation of the machine. Whereas a single ground fault close to the neutral point may not cause any immediate danger to the machine, the occurrence of the next ground fault will generate large circulating currents that can produce severe damage. To limit ground current at a single ground fault near the terminals, the machine is often grounded via an impedance means to limit mechanical and thermal stresses, thus reducing the resultant damage to the machine. Such a grounding system is often used to provide a means for detecting ground faults within the machine.
In principle, existing systems for detecting ground faults in electrical machines are based on the measurement of an electrical quantity, for example, measured current and/or voltage values. A reference value is predefined for the measured values of the electrical quantity. A ground fault is detected based on the measured values and the predefined reference value. Alternatively, other electrical values can be estimated based on the measured values and the detection of the ground fault is made based on the estimated values and a corresponding predefined reference value. An alarm or a trip signal may be initiated when a ground fault is detected.
U.S. Pat. No. 4,851,766 discloses such a system, wherein, an impedance value is calculated based on measured current and voltage and a ground fault is detect by comparing the impedance value with a predefined reference value.
However, measured values can be significantly different under different machine conditions/states. This means that either such a method can only be applied for detecting a ground fault when the machine is in a specific status or it may conduct a false alarm or trip signal due to an inaccurate detection. Therefore, the sensitivity of a system based on this invention is reduced.
U.S. Pat. No. 6,172,509 discloses a system for detecting turn-to-turn and other winding faults in a polyphase alternating current machine, wherein measured current values are normalized. The normalization of the measured currents accounts for inherent machine imbalance and varying load. The fault detection is determined by identifying current flow changes, which however requires that several independent circuits be measured simultaneously.
U.S. Pat. No. 5,508,620 (D1) describes methods for determining ground faults in an electrical machine. It describes two different circuits in FIGS. 1a and 1b for detecting a ground fault at a stator and rotor, respectively, of an electrical machine. The fault resistance are different for the different circuits (column 4, line 5-10, and column 4, line 31-33). The faults are related to different fault resistances depending on which fault circuit and fault method is used.